The present invention relates to an apparatus for determining base sequence of nucleic acid, and particularly to an apparatus for determining base sequence of nucleic acid at high speeds maintaining a high sensitivity by detecting the beta rays emitted from a nucleic acid that is labeled by radio isotope.
So far, the base sequence of nucleic acid has generally been determined relying upon the Maxam-Gilbert method. According to this method, a nucleic acid labeled by radio isotope is chemically cut into fragments, and the fragments of nucleic acid having different lengths are arranged in the order of molecular weights in a gel support sandwiched between glass plates. The gel support is then peeled off from the glass plates, an autoradiogram thereof is photographed to detect an electrophoretic band which includes fragments of radioactive nucleic acid, and whereby the base sequence of nucleic acid is determined.
A conventional method of determining base sequence of nucleic acid fragments based upon the Maxam-Gilbert method will be described below with reference to the drawings. FIG. 1 is a perspective view showing the structure of a conventional electrophoresis apparatus for nucleic acid fragments, which consists of a gel 2 which is sandwiched between two pieces of glass plates 3 and which separates migration of nucleic acid fragments, electrode solution vessels 1a and 1b in which are immersed both ends of the gel 2 which separates migration, and a d-c power source 6. A sample of nucleic acid fragment labeled by radio isotope (e.g., .sup.32 P) is supplied to a slot 5 on the negative pole side of the gel 2 for separating migration, and is allowed to migrate at a voltage Ev of about 40 V/cm per a gel length. Nucleic acid fragments having the same molecular weight form electrophoretic bands 4 heading from the negative pole toward the positive pole, and migrate at migration speeds nearly in reverse proportion to the logarithms of molecular weights. Sequence of nucleic acid bases in the order of molecular weights is determined from the migration pattern of electrophoretic bands 4 of nucleic acid fragments.
In order to read the pattern in this case, it is a generally accepted practice to take a picture of the autoradiogram as mentioned already. With the autoradiogram, however, since the nucleic acid labeled by radio isotope exists in a very small amount, an extended period of time (more than 50 hours) is necessary to transfer it onto an X-ray film, making it difficult to quickly measure various kinds of samples.
In order to compensate the above-mentioned problem of the autoradiogram, it has been contrived to measure the samples at high speeds by directly detecting beta rays from the nucleic acid fragments. FIG. 2 is a section view of the electrophoresis apparatus for this purpose. Namely, beta rays are detected at a moment when the nucleic acid fragment 4 that is migrating passes through the detectors 7, 8, in order to measure the same at a high speed without using any autoradiogram. According to this method, as will be understood from FIG. 2, beta rays incident on the detector 7 of one side are detected by the detector 7 only, and an extended period of time is required to take a measurement maintaining a high S/N ratio. By using two detectors 7, 8 to take measurements from both sides of the glass plates, furthermore, the amount of incident beta rays is doubled. However, beta rays in a direction perpendicular to the surface of the paper are not measured.